Hybrid electric vehicles are widely viewed as being among the most practical of the low-polluting vehicles. A hybrid electric vehicle includes an electric "traction" battery which provides electric power for an electric traction motor, which in turn drives the wheels of the vehicle. The "hybrid" aspect of a hybrid electric vehicle lies in the use of a secondary or supplemental source of electrical energy for recharging the traction battery during operation of the vehicle. This secondary source of electrical energy may be solar panels, a fuel cell, a generator driven by an internal combustion engine, or generally any other source of electrical energy. When an internal combustion engine is used as the secondary source of electrical power, it commonly is a relatively small engine which uses little fuel, and produces little pollution. A concomitant advantage is that such a small internal combustion engine can be operated within a limited RPM range, so that pollution controls of the engine may be optimized. The terms "primary" and "secondary" when used to describe the sources of electrical energy merely relate to the way energy is distributed during operation, and are not of fundamental importance to the invention. A simple electrically driven vehicle powered only by electrical batteries has the disadvantages that the batteries may become depleted while the vehicle is far from a battery charging station, and even when such a vehicle successfully returns to its depot after a day's use, the batteries must then be recharged. The hybrid electric vehicle has the significant advantage over a simple electrically powered vehicle that the hybrid electric vehicle recharges its own batteries during operation, and so should not ordinarily require any external battery charging. Thus, the hybrid electric vehicle can be used much like an ordinary vehicle powered by internal combustion engines, requiring only replenishing of the fuel. Another major advantage of the hybrid electric vehicle is its good fuel mileage. The advantage in fuel mileage arises from the use of regenerative dynamic braking, which converts kinetic energy of motion into electrical power during at least a portion of braking, and returns the energy to the battery. It has been found that braking losses account for somewhere near half of all the frictional losses experienced by a vehicle in an urban transit setting. The recovery of this 50% of energy, and returning it to the batteries for further use, permits the use of a much smaller "secondary" fuel-operated electrical generator than would be the case if regenerative braking were not used. In turn, the smaller secondary electrical source results in less fuel used per unit time, or per mile. Yet another advantage of a hybrid electric vehicle is that under many conditions, the power which is available for accelerating the vehicle is the sum of the maximum power which can be supplied by the batteries plus the maximum power which can be generated by the secondary electrical generator. When the electrical generator is a diesel-powered internal combustion engine, the combination of the battery power and the diesel power can result in a total motive force which is quite substantial, notwithstanding the good fuel mileage.
While hybrid electric vehicles are economically and environmentally advantageous, they must be somewhat "foolproof", in that they must be similar to conventional internal-combustion-powered vehicles, in their operation and in their responses to operator input, in order to achieve widespread acceptance.